As defined by ASME Y14.24, an Altered Item Drawing does exactly what its name says: it modifies some other part by adding and/or subtracting features. The original part is usually under the configuration control of some other design activity (usually a different company) or is the subject of some national standard. This drawing type is identifying because the resulting part might no longer be interchangeable with the part from which it was created, and the original part is absolutely not interchangeable with it.
Contrast with Selected Item Drawing and Make-from Drawing. To really bake your noodle, see Multi-Purpose Drawing. See also the CI Development Cycle (Intermediate).